1. Field of the Invention
The present invention relates to glass having a high refractive index that is suited to use in the lenses employed in cameras and projectors. The present invention further relates to optical elements made from this glass and to a method for manufacturing such optical elements.
2. Discussion of the Background
Highly refractive glass with a refractive index of 1.8 or more can be divided into three types: high dispersion glass having an Abbé number of 25 or less, medium dispersion glass with an Abbé number of 25 to 35, and low dispersion glass with an Abbé number of 35 or more. Conventionally, medium dispersion glass with an Abbé number of 25 to 35 that contains lead is known. However, due to concern for the environment and humans, the following glasses that achieve the above-stated optical characteristics without the incorporation of lead have been proposed in recent years.
For example, Japanese Unexamined Patent Publication (KOKAI) Showa No. 62-275038, which is expressly incorporated herein by reference in its entirety, discloses a glass having a refractive index nd of 1.8 to 1.85, an Abbé number v(nu)d of 31 to 32, and a specific gravity of 3.0 to 3.7 that is desirable for use in eyeglasses.
Japanese Unexamined Patent Publication (KOKAI) Heisei No. 7-41334, which is expressly incorporated herein by reference in its entirety, discloses a glass with a refractive index of 1.84 to 1.93, an Abbé number nud of 25 to 32, and a specific gravity of 4.0 or less.
Japanese Unexamined Patent Publication (KOKAI) No. 2000-128570, which is expressly incorporated herein by reference in its entirety, discloses a glass having a refractive index of 1.84 or above and a specific gravity of 3.9 or less.
Japanese Unexamined Patent Publication (KOKAI) No. 2004-175632, or English language family member U.S. patent application No. 2004-220041 AA, which are expressly incorporated herein by reference in their entirety, discloses a glass having a refractive index of 1.70 to 1.93 and an Abbé number nud of 28 to 45.
In the manufacturing of high-quality optical elements in which glass is softened by heating, when the glass serving as starting material does not melt readily, requiring that the melting temperature be raised, the platinum comprising the melting container ends up melting into the glass, discoloring the glass.
Further, when molding glass with low devitrification stability while in a molten state, the glass ends up devitrifying as the glass melt is quenched. Further, when devitrification stability is only good during molding of the glass melt and the molded glass is subsequently employed as a material, being reheated and melted during molding, low devitrification stability causes the molded product to devitrify.
Thus, obtaining a high-quality optical element requires the use, from melting to manufacturing of the optical element, of a glass that simultaneously satisfies the requirements of meltability, devitrification stability when molding glass in a molten state, low coloration, and devitrification stability in molded glass that has been softened by heating.
The glasses disclosed in the above-mentioned four patent publications are characterized by not containing Harmful Pb. However, the glasses have drawbacks that they do not satisfy simultaneously the requirements of meltability, devitrification stability when molding glass in a molten state, low coloration, and devitrification stability in molded glass that has been softened by heating. Further, the various above-described glasses all contain an essential component in the form of TiO2, and thus present the problem of increased coloration, which is thought to be the result of titanium ions generated by high temperature melting.